Paradigms that provide independent input to each eye (e.g. binocular rivalry) have been used to test the role of subcortical visual processing streams and establish the boundaries of visual awareness. These methods have advantages over backward masking, which is insufficient for complete disruption of the ventral visual pathway. The current fMRI study presented images of faces and houses that were rendered subliminal via binocular rivalry combined with flash suppression and an orthogonal task – with the ultimate objective of examining subcortical pathways involved in the perception of social stimuli.

During fMRI data collection, 12 young adult participants wore anaglyph glasses and viewed centrally presented supraliminal words on a sharply moving checkerboard. Participants identified the first letter of each word as a consonant or vowel. Fearful faces and houses were presented to the non-dominant eye and suppressed from conscious awareness. Catch trials determined if and when participants perceived the subliminal stimuli; only data acquired prior to onset of awareness were analyzed.

Whole-brain, mixed-model GLM analyses found significantly greater activation for subliminal faces versus subliminal houses in precuneus and left inferior parietal cortices. An a priori ROI analysis of bilateral amygdalae revealed a significantly greater left amygdala response for subliminal faces. Psychophysiological interaction (PPI) analyses of individually-defined left amygdala showed task-dependent correlations with bilateral pulvinar and early visual cortices. Previous findings have implicated the amygdala and pulvinar in subliminal threat and saliency detection, respectively. While spatial resources are typically recruited in supraliminal vision, these data suggest that precuneous and parietal cortices are activated prior to social stimulus awareness. We suggest this response to detection of environmentally relevant stimuli also serves a preparatory role in spatial resource allocation for subsequent behavior. Ultimately, present data cast some doubt on the distinction typically made between subcortical and cortical pathways in subliminal perception of social stimuli.